A proposed communication system known as evolved universal terrestrial radio access network (E-UTRAN, also referred to as UTRAN-LTE or 3.9G) is currently under discussion within the 3GPP. The current working assumption is that the downlink DL access technique will be orthogonal frequency division multiple access OFDMA, and the uplink UL technique will be single-carrier frequency division multiple access SC-FDMA.
Reference can be made to 3GPP TR 25.814, V7.0.0 (2006-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA) (Release 7), such as generally in section 9.1 for a description of the SC-FDMA UL of UTRAN-LTE.
This invention relates to the UL part of the UTRAN-LTE, specifically to the detection of a non-synchronized random access channel RACH preamble. As will be seen, two competing concerns with RACH preamble detection are missed detection and false alarms.
As is described in Section 9.1 of 3GPP TR 25.814, the basic uplink transmission scheme is single-carrier transmission (SC-FDMA) with cyclic prefix to achieve uplink inter-user orthogonality and to enable efficient frequency-domain equalization at the receiver side. Frequency-domain generation of the signal, sometimes known as discrete Fourier transform DFT-spread OFDM (DFT S-OFDM). allows for a relatively high degree of commonality with the DL OFDM scheme and the same parameters, e.g., clock frequency, can be reused.
In 3GPP there has been extensive discussions related to the non-synchronized RACH preamble structure. A constant-amplitude zero autocorrelation (CAZAC) sequence, specifically a Zadoff-Chu sequence, has been agreed to be the preamble sequence for LTE UL. Its ideal periodic autocorrelation properties have been seen as beneficial for a RACH preamble, for example, multiple preambles are obtained from a single base Zadoff-Chu sequence with cyclic shifts of the sequence. Zadoff-Chu sequences of odd length are given by
                    a        u            ⁡              (        k        )              =          exp      (                        -          j                ⁢                                  ⁢        2        ⁢                                  ⁢        π        ⁢                                  ⁢        u        ⁢                              qk            +                                          (                                                      k                    2                                    +                  k                                )                            /              2                                                                                  ⁢                          N              G                        ⁢                                                                    )        ,where k is the sample index, q is an integer and u is the sequence index that defines the base sequence, and NG is the length of the Zadoff-Chu CAZAC sequence. Generally, NG and u are restricted to be co-primes. In the following,au,d(k)=au(k−d mod NG)refers to the dth cyclic shift of sequence au.
The current working assumptions in 3GPP are that the RACH preamble is transmitted on a 1.08 MHz bandwidth, 64 preambles are used in each cell, and total duration of the preamble is 0.8 ms, excluding any cyclic prefix CP and guard time. In 3GPP-LTE, it is expected that the preamble will include a single 0.8 ms Zadoff-Chu sequence or a 0.8 ms Zadoff-Chu sequence repeated twice. The repeating ZC sequence is to improve RACH coverage. A repetition of a 0.4 ms Zadoff-Chu sequence was also considered in 3GPP. In the following, the duration of the Zadoff-Chu sequence is marked with Ts. Preambles with single and repeating ZC sequences are shown respectively at FIGS. 2A and 2B.
Alternative ways of generating the single carrier preamble signal are shown by the process blocks of FIGS. 2C-E. FIG. 2C shows time domain processing and FIGS. 2D-2E show frequency domain processing, which are seen as more likely to be adopted in UTRAN-LTE. The processing in FIG. 2D is accordance with the DFT-S-OFDMA system used for transmission on the scheduled resources, while FIG. 2E simplifies processing compared with FIG. 2D. The filtering-blocks in FIGS. 2D-2E may or may not be included. In each of FIGS. 2C-E, the ZC sequence is generated 201, the cyclically shifted 202 before a cyclic prefix is inserted 203. FIG. 2C follows CP insertion by channel filtering 204 prior to transmitting 205. In FIG. 2D, a discrete Fourier transform DFT is executed 206 on the cyclically shifted ZC sequence, the DFT results are filtered 207, and mapped 208 to the subcarriers. Prior to CP insertion 203, an inverse DFT is executed 209 where the subcarriers are spaced, and then the SP is inserted 203 prior to transmission 205. FIG. 2E differs from FIG. 2D in that there is no DFT 206, and the cyclic shifting 202 takes place after the IDFT block 209.
It has been noted that the correlation properties of Zadoff-Chu sequence deteriorates significantly with a large frequency offset between the user equipment UE and the Node B (e.g., base station BS) transceivers. When correlating the received, frequency shifted signal with a replica of the transmitted ZC sequence, there also appear side correlation peaks which can be even larger than the main correlation peak. The relative location of the side correlation peaks depends on the sequence index u. Cyclic shifts of a Zadoff-Chu sequence are used in different preambles, but the false detection rate or false alarm rate increases with increasing frequency offset between those different preambles. (A false alarm is considered to be where a preamble transmission triggers detection of also another preamble.) The main correlation peak also diminishes with increasing frequency offset, which increases the probability of a missed detection.
The problems in the preamble detections due to frequency offset from the sequence used in other preambles can also be summarized as follows:                NG cyclic shifts of a Zadoff-Chu sequence can be seen as an orthogonal base of NG-dimensional space.        A frequency offset of 1/Ts rotates the transmitted sequence from the original direction to the direction of another cyclic shift B. As a result, the received sequence is orthogonal with the transmitted one. The cyclic shift B depends on the sign of the frequency offset as well as on the u-index of the sequence.        When frequency offset is less than 1/Ts, the rotation is not restricted to the plane defined by the original sequence and the cyclic shift B. However, the largest components of the rotation are along these directions.A specific preamble mode, restricted sets of cyclic shifts, where the cyclic shifts used as preambles are selected according to suitable constraints is introduced in 3GPP as a solution to this sensitivity to frequency offset. Details in this regard can be seen at co-owned application PCT/IB2007/004134, filed on Dec. 2, 2007 and entitled “Apparatus, methods and Computer Program Products providing Limited Use of ZADOFF-CHU Sequences in Pilot or Preamble Signals”. In the following, we consider the detection of preambles from unrestricted sets of cyclic shifts.        
In view of FIGS. 2A-2B, there are then two distinct cases. For brevity, we term repeating a Zadoff-Chu sequence in a preamble as case 1; and we term combining partial correlations of a Zadoff-Chu sequence that is not repeated in a preamble as case 2.
Consider case 1. The coherent and non-coherent combining of the repeated sequence correlations are obvious methods. The benefits of coherent combining over non-coherent combining include:                Better detection probability at low SNR (Ep/No) for low velocity terminals, and        Less false alarms due to high velocity terminals.The drawback is        High missed detection probability for high velocity terminals.        
High velocity terminals are a distinct concern because the Doppler effect between a fast-moving UE and a BS affect the frequency at which a CAZAC sequence preamble is received, and therefore its frequency offset. As would be expected, the benefit of non-coherent combining as compared to coherent combining is the opposite, to whit:                Better detection probability for high velocity terminals;and the drawback is        High false alarm rate due to high velocity terminals.        
With a minor amount of increased computational complexity, both coherent and non-coherent combinations can be calculated and further combined with selection or soft combining. This approach is evident in a paper sourced to Texas Instruments entitled: “3GPP TSG RAN WG1 #47, Riga, Latvia, Nov. 6-10, 2006; Non Synchronized Random Access Design for High Doppler Conditions”. The method described there achieves good detection probabilities for both high and low velocity terminals. The false alarm properties depend on the used soft combining method, but relatively high false alarm rates are expected. This is the case at least for hard combining.
For case 2, partial correlation of a single (long) sequence and non-coherent combination of these partial correlations is well-known in the art as a general proposition.
What is needed in the art is a way to reduce rates/probabilities for both false alarms and missed detections. Preferably, such an approach would show improvements where the preamble has a single CAZAC sequence and a repeated CAZAC sequence. It is noted that the invention is not limited to 3.9G systems, generalized CAZAC or more specific Zadoff-Chu sequences; those are used as non-limiting examples to illustrate the inventive detection techniques and apparatus.